In association with the trend toward smaller-size, lighter-weight, and thinner design of portable equipment, as represented by portable telephone, note-book type personal computers, and camcorders, there has been remarkable progress in recent years in the small-size secondary batteries to be used as the power source of such equipment. Following the initially commercialized conventional type lead-acid system and nickel-cadmium system, new types of nickel-metal hydride systems and lithium-ion systems with higher energy densities have recently been commercialized.
With small-size sealed lead-acid batteries, rectangular batteries are the generally adopted configuration. In such batteries, the electrode groups are constructed by alternately stacking a plurality of positive plates and negative plates with a separator interposed and connecting together the electrode plates of the same polarity. The groups of electrodes then are encased in a 3-cell or 6-cell mono-block type plastic battery container, and are sealed after being connected in series.
Nickel-cadmium batteries and nickel-metal hydride batteries are used in the form of a battery pack in which a plurality of either cylindrical cells, obtained by encasing and sealing an electrode group constructed by spirally winding one each of strip-shaped positive plate and strip-shaped negative plate with a separator interposed in a metallic cell container, or rectangular cells, obtained by encasing and sealing an electrode group constructed by alternately stacking a plurality of positive plates and negative plates with a separator interposed and connecting together the electrode plates of the same polarity in a metallic container, are connected in series and/or in parallel in order to obtain a required voltage and capacity.
With lithium-ion secondary batteries, cylindrical cells are constructed in basically the same way as the nickel-cadmium system batteries and nickel-metal hydride system batteries. In contrast to this, with thin type flat cells having a rectangular or oval cross section and which are regarded important from the standpoints of thinning appliances and reducing dead space of the power source, because the thicknesses of the positive and negative plates and the separator are extremely small, a cell configuration is adopted in which an electrode group is constructed by spirally winding one each of strip-shaped positive and negative plates with a separator interposed in such a manner that they are folded to make the cross section oval in shape. The electrode groups are encased in a container, and sealed after pouring and impregnating with an electrolyte.
In order to further make this type of a cell thinner or higher in capacity, the core material of the electrode plates is made as thin as possible and the amount of the separator in the core portion and the outermost portion of the electrode group is reduced, or the thickness of the electrode group is made smaller by applying a higher pressure to the electrode group when it is inserted and encased in a cell container.
On the other hand, in order to fabricate such an electrode group with a high productivity, a core portion is formed as a first step by winding a separator while keeping it firmly held on a mandrel, a coil one each of strip-shaped positive plate and negative plate is spirally wound with the separator interposed, and the outer surface is fixed by wrapping with an extra length of the separator, thereby completing an electrode group having an elliptical cross section. As a second step, a process is carried out in which the electrode group is pressed and deformed while being sandwiched between a pair of flat plates which are parallel to the major axis of the electrode group and have an elliptical cross section, thereby producing an electrode group having an oval cross section as shown in FIG. 1. FIG. 1 is an enlarged cross-sectional view of an essential part of an electrode group 1 of a thin-type lithium-ion secondary flat cell. In FIG. 1, the electrode group 1 is constructed by spirally winding one each of strip-shaped positive plate 11 and negative plate 12 in such a way that they are folded with a separator 13 interposed. The electrode group 1 is made by first spirally winding the positive plate 11 and the negative plate 12 with the separator 13 interposed enwrapping a core portion 13a which is made by folding over the tip of one end of the separator 13. The separator 13 enwraps the outside of the electrode group 1, and the tip 13b of the other end of the separator is tightly fastened to the electrode group 1 by a heat-sealing method or the like.
In the final process of pressing and deforming in the second step of constructing an electrode group, breakage of an electrode plate sometimes takes place because of tearing off of the core material of the electrode plate at the innermost folded portion 11a of the positive plate 11 close to the core portion 13a and the subsequent folded portion 11a', and at the innermost folded portion 12a and subsequent folded portion 12a' of the negative plate 12, thus causing a reduction in the cell capacity. Also, even when an electrode plate is not broken, the active material layer sometimes peels off from the core material, leading to dropping of small portions of the active material layer, thus damaging the separator and causing internal short circuits between the positive and negative electrodes, resulting in a loss of reliability in the lithium-ion secondary batteries employing an organic electrolyte. Consequently, it is critically important to totally prevent such breakage of electrode plates and/or peeling off and dropping of the active material layer. Many of such failures tend to take place with the positive plate which has a relatively higher filling density of the active material layer than the negative plate and which uses as the core material of the electrode plate aluminum which has a relatively lower tensile strength and repetitive bending strength than the copper foil used as the core material of the negative plate. Also, the failures tend to take place on the innermost folded portion which is close to the core portion.
FIG. 2 is a plan view of a folded portion of a strip-shaped positive plate of an electrode group having an oval cross section as constructed in the final step. In FIG. 2, breakage and/or peeling off and dropping of the active material layer tends to take place at the innermost folded portion 11a of the positive plate 11 which is close to the core portion. Occasionally, breakage of the positive plate and/or peeling off and dropping of the active material layer takes place at the folded portion 11a' which is next to the innermost folded portion 11a. No breakage of the positive plate or peeling off and dropping of the active material layer takes place in the subsequent folded portion 11a" (corresponding to the outer region of the innermost folded portion 11a). Reference numeral 11b is a positive plate lead tab made of aluminum onto which an insulating tape 11c is affixed.
FIG. 3 is an enlarged cross-sectional view of the core portion and its outer regions only of an electrode group having an oval cross section. In FIG. 3, the negative plate 12, which is adjacent to the core portion 13a consisting of a separator 13 that separates the positive plate 11 and the negative plate 12, comprises a core material 12d consisting of a copper foil onto both sides of which active material layers 12e and 12f, mainly consisting of carbon material, are coated. At very rare intervals, peeling off and dropping of the active material layer 12e' on the inner side of the innermost folded portion of the negative plate have been observed. Compared with this negative plate, the positive plate 11 comprises a core material 11d consisting of an aluminum foil onto both sides of which active material layers 11e and 11f, mainly consisting of lithium cobaltate (LiCoO.sub.2), are coated. Even though the positive plate 11 has been placed on the outside of the innermost negative plate 12 with the separator 13 interposed, many instances of peeling off and dropping of the active material layer 11e' on the inner side and the active material layer 11f' on the outer side have been observed at the initial folded portion, and breakage of the positive plate 11 has taken place due to tearing of the core material 11d at the folded portion.
In order to prevent the above-described failure due to breakage of the electrode plate and/or peeling off and dropping of the active material layers, measures such as increasing the thickness of the core material of the electrode plate, increasing the radius of curvature of the folded portion of the electrode plate by making the core portion consisting of the separator of the electrode group, or preliminarily removing the active material layers in the vicinity of the folded portion of the electrode plates have been suggested. However, each such measure inevitably leads to a reduction in the battery capacity, and is not an appropriate approach for a new type of battery aiming at achieving higher energy densities.
Japanese Laid-Open Patent Applications No. Sho 60-133655 and No. Hei 5-41211 disclose a cylindrical cell in which a coil-shaped electrode group is constructed by forming parallel grooves in the direction perpendicular to the direction of spirally winding a foamed electrode plate obtained by filling an active material onto a spongy nickel sheet or a paste type electrode plate in which active material paste has been coated on a core material and dried.
However, no proposal has heretofore been made which is effective to suppress failures due to breakage of electrode plates and/or peeling off and dropping of the active material layers when constructing an electrode group having an oval cross section for a thin type flat cell comprising strip-shaped positive and negative plates that use a foil-form core material and a separator.